Damage to the upper atmosphere caused by the emission of fluorocarbons won't likely return to the level it was at in 1980 until 2065, according to scientists speaking at the American Geophysical Union, an annual earth sciences conference taking place in San Francisco. Earlier, researchers anticipated that the hole would return to 1980 levels in 2050.

The extra 15 years mean that ultraviolet rays, blocked by ozone in the upper atmosphere, will become a larger health problem for a longer period of time than expected.

"A delay means that ozone will remain lower in later years," said Paul Newman, an atmospheric scientist with the NASA Goddard Space Flight Center. "UV levels will be slightly higher in later years."

The culprit in the delay appears to be lingering use of ozone-depleting chemicals, and equipment that contain these chemicals, in developed nations. The Montreal Protocol, signed in 1987 and enacted by laws in various nations, banned the production and importation of fluorocarbons and compounds like methyl bromide in developed nations. Participating emerging nations, meanwhile, put themselves on a program to phase out use of these chemicals.

The treaty, however, didn't regulate the continued use of stockpiles that already existed in the late '80s, when the Montreal Protocol was signed. Those chemicals produced almost 20 years ago continue to leak out and subsequently react with the atmosphere.

"The bulk of them are in equipment--fire extinguishers, air conditioners and refrigerators. These are produced, but unemitted, reservoirs," said Dale Hurst, a research associate with the Earth Systems Research Laboratories at the National Oceanographic and Atmospheric Administration (NOAA).

On the positive side, the maximum depletion of the ozone should occur between 2000 and 2010 before a slow recovery begins, according to John Austin, a visiting scientist at NOAA's Geophysical Fluid Dynamics Laboratory.

Hurst added that atmospheric sampling for chemicals with low-flying airplanes above North America make it fairly certain that these gasses continue to emit from the U.S. and Canada.

When gaseous fluorocarbons such as chlorine nitrate or methyl bromide escape into the atmosphere, they eventually circulate toward the poles. There, the cold air causes a chemical reaction that frees the chlorine and bromine. These chemicals then react with ozone, a molecule that consists of three oxygen atoms, and hence eat away the ozone layer. Technically, a hole doesn't emerge, but 50 percent or more of the ozone in a given region vanishes, letting UV rays through.

Bromine destroys ozone more efficiently. However, it also cycles through the global environment much more rapidly, raining back to earth in about eight months. "These kinds of chemicals are easier to deal with," said Hurst.

By contrast, fluorocarbons can linger for years before breaking down.

Atmospheric conditions play a role in how rapidly the ozone declines. The ozone hole is larger above Antarctica because the air is colder. In the Arctic, high winds combined with warmer temperatures have lead to a smaller hole.

While the atmosphere plays a role, there is little debate that the ozone holes are primarily caused by chemicals. Newman, who discounted the role of industrial chemicals in the '80s, said he was wrong.

"There is no question in the scientific community about the role of chlorine and bromine," Newman said.